[Drake] L7 and L4 PSU REPLIES

[GW4BLE]

My thanks to the many who replied to my recent posting. As some were direct
emails, here are a cut'n'paste selection of the responses....

Yep, it's the same as years ago I used an L4 ps to use with an L7 amp.

Is it possible and have you tried to tighten the laminations?

As transformers warm up the laminations something start to vibrate if they
aren't tightly compressed.  Perhaps if the transformer is one which has the
obvious not on the end of a bolt to tighten it might be worthwhile seeing it
that makes a difference.

I think with heat things expand and any potting lacquer that is keeping
things tight softens.

As far as I can tell, the power supplies are identical and I have both units
here.

Since there is no problem with the L-4 PS, that eliminates any problem in
the amplifier.

I believe what you are experiencing is loose laminations in the transformer.
When the transformer heats up, the heat generated causes the laminations to
expand and the hum disappears since everything is nice and snug. You might
try tightening up the hardware on the tranny frames. That may solve the
problem.  If you haven't checked the filters lately, now is a good time to
do that while everything is apart.

SAFETY FIRST:  Be sure to disconnect the power supply from the AC line and
discharge the caps before getting in there with your bare hands.

I have a fairly early L7 psu here and, when I checked, they just seem to
have changed the name rather than make any alterations. It still uses the
same cr*p diodes that are in the L4-psu.

 Hello
They are the same.

Going by memory,  I believe both power supplies are virtually the same.
There may be some minor differences with the diode strings such as
equalizing resistors across each diode (which are not necessary) but
nothing major.

Try a cooling fan on the powersupply.   Worked for me..

I cannot swear to it but I believe the power supply for the L4B and the L7
Drake amplifiers is the same.  I have used one supply with another amp and
visa versa for test purposes.
You might check to see if all the bolts that hold the transformer together
and to the chassis are tight.  If one is loose and the transformer heats up,
that could be the problem, especially if the buzz changes with load to the
amplifier.


73

Steve
GW4BLE